Component bindings in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a component binding in Svelte?

A component binding in Svelte connects a variable in a parent component to a property or value inside a child component, allowing two-way data flow.

beginner

How do you bind a variable to an input element in Svelte?

Use the bind:value directive on the input element, like <input bind:value={name} />, to keep the variable name and the input's value in sync.

intermediate

What happens when you use bind:this in Svelte?

The bind:this directive assigns a reference of the DOM element or component instance to a variable, letting you interact with it directly in your script.

intermediate

Can you bind to custom component props in Svelte? How?

Yes, by using bind:propName on the child component tag, you can create two-way binding between the parent variable and the child's prop.

beginner

Why is two-way binding useful in Svelte components?

Two-way binding keeps data synchronized between parent and child components automatically, reducing the need for manual event handling and making UI updates simpler.

Which directive is used to bind a variable to an input's value in Svelte?

A bind:model

B bind:input

C bind:data

D bind:value

The correct directive is bind:value to connect a variable to an input's value.

What does bind:this do in a Svelte component?

A Binds a variable to the DOM element or component instance

B Binds a variable to the component's CSS class

C Binds a variable to the component's event

D Binds a variable to the component's style

bind:this assigns a reference of the DOM element or component instance to a variable.

How do you create two-way binding with a custom prop in a child component?

A Use bind:custom on the child component tag

B Use bind:propName on the child component tag

C Use bind:this on the child component tag

D Use bind:value on the child component tag

Two-way binding with custom props uses bind:propName on the child component.

What is a benefit of using component bindings in Svelte?

A It disables user input

B It makes the code run faster

C It automatically synchronizes data between components

D It removes the need for CSS

Component bindings keep data synchronized automatically between components.

Which of these is NOT a valid use of binding in Svelte?

A bind:style on a component

B bind:this on a DOM element

C bind:value on an input element

D bind:propName on a child component

bind:style is not a valid binding directive in Svelte.

Explain how two-way binding works in Svelte components and give an example with an input element.

Think about how typing in an input updates a variable and vice versa.

Describe the purpose of bind:this in Svelte and when you might use it.

Consider when you need to call a method or focus an element directly.

Practice

1. What does bind:value do in a Svelte component?

easy

A. It creates a two-way connection between a variable and an input's value.

B. It only sets the initial value of the input without updating.

C. It listens for clicks on the input element.

D. It disables the input field.

Solution

1. Step 1: Understand bind:value usage

   In Svelte, bind:value links a variable to an input's value so changes update both ways.

2. Step 2: Compare options with behavior

   Only It creates a two-way connection between a variable and an input's value. describes this two-way syncing correctly; others describe unrelated behaviors.

3. Final Answer:

   It creates a two-way connection between a variable and an input's value. -> Option A

4. Quick Check:

   Two-way binding = It creates a two-way connection between a variable and an input's value. [OK]

Hint: Remember: bind:value syncs input and variable both ways [OK]

Common Mistakes:

• Thinking bind:value only sets initial value
• Confusing bind:value with event listeners
• Assuming bind:value disables input

2. Which of the following is the correct syntax to bind a parent's variable count to a child's prop value in Svelte?

easy

A.

B. count

C.

D.

Solution

1. Step 1: Recall Svelte binding syntax

   To bind a parent's variable to a child's prop, use bind:prop={variable} syntax.

2. Step 2: Evaluate options

   <Child bind:value={count} /> matches correct syntax. <Child value={count} /> passes value one-way. Options B and D have invalid syntax.

3. Final Answer:

   <Child bind:value={count} /> -> Option D

4. Quick Check:

   Correct bind syntax = <Child bind:value={count} /> [OK]

Hint: Use bind:prop={variable} to sync parent and child [OK]

Common Mistakes:

• Using curly braces without bind:
• Trying to bind without ={}
• Passing value as child content instead of prop

3. Given this Svelte code:

<script>
  let name = 'Alice';
</script>

<Child bind:name />

<!-- Child.svelte -->
<script>
  export let name;
</script>

<input bind:value={name} />

What happens when the user types 'Bob' in the input?

medium

A. An error occurs because bind:name is invalid.

B. Only the child's name updates; parent stays 'Alice'.

C. The parent's name variable updates to 'Bob' automatically.

D. The input value resets to 'Alice' immediately.

Solution

1. Step 1: Understand two-way binding between parent and child

   The parent binds name to child's name prop with bind:name, syncing both variables.

2. Step 2: Input binding updates child's name and propagates to parent

   Typing in input updates child's name, which updates parent's name automatically.

3. Final Answer:

   The parent's name variable updates to 'Bob' automatically. -> Option C

4. Quick Check:

   Two-way bind updates parent and child = The parent's name variable updates to 'Bob' automatically. [OK]

Hint: bind:prop syncs parent and child variables both ways [OK]

Common Mistakes:

• Thinking only child updates
• Expecting input to reset on typing
• Believing bind:name is invalid syntax

4. Identify the error in this Svelte code snippet:

<script>
  let age = 30;
</script>

<Child bind:age></Child>

<!-- Child.svelte -->
<script>
  export let age;
</script>

<input value={age} />

medium

A. The child input should use bind:value={age} for two-way binding.

B. The parent variable age must be a string, not a number.

C. The child component must not export age to bind it.

D. The parent should use bind:age={age} instead.

Solution

1. Step 1: Check child input binding

   The child input uses value={age}, which is one-way binding only.

2. Step 2: Fix to two-way binding for sync

   To sync changes back to parent, child input must use bind:value={age}.

3. Final Answer:

   The child input should use bind:value={age} for two-way binding. -> Option A

4. Quick Check:

   Two-way input binding needed = The child input should use bind:value={age} for two-way binding. [OK]

Hint: Use bind:value on inputs for two-way sync, not value= alone [OK]

Common Mistakes:

• Using value= instead of bind:value= on inputs
• Thinking export let disables binding
• Misusing bind:age syntax on parent

5. You want to create a custom slider component that updates a parent's volume variable as the slider moves. Which approach correctly uses Svelte component bindings to achieve this?

hard

A. In parent: <Slider value={volume} />In Slider.svelte: export let value; and <input type='range' value={value} />

B. In parent: <Slider bind:value={volume} />In Slider.svelte: export let value; and <input type='range' bind:value={value} />

C. In parent: <Slider bind:volume={volume} />In Slider.svelte: export let volume; and <input type='range' bind:value={volume} />

D. In parent: <Slider bind:value />In Slider.svelte: let value; and <input type='range' bind:value={value} />

Solution

1. Step 1: Understand binding prop names

   The parent binds its volume variable to child's value prop using bind:value={volume}.

2. Step 2: Child exports value and binds input's value to it

   The child exports value and uses bind:value={value} on the input for two-way sync.

3. Step 3: Check other options for errors

   In parent: <Slider value={volume} />
   In Slider.svelte: export let value; and <input type='range' value={value} /> uses one-way binding only. In parent: <Slider bind:volume={volume} />
   In Slider.svelte: export let volume; and <input type='range' bind:value={volume} /> mismatches prop names. In parent: <Slider bind:value />
   In Slider.svelte: let value; and <input type='range' bind:value={value} /> misses binding variable in parent.

4. Final Answer:

   Parent binds volume to child's value; child exports value and binds input value. -> Option B

5. Quick Check:

   Matching bind names and two-way input binding = In parent: <Slider bind:value={volume} />
   In Slider.svelte: export let value; and <input type='range' bind:value={value} /> [OK]

Hint: Bind parent's variable to child's exported prop with same name [OK]

Common Mistakes:

• Mismatching prop names between parent and child
• Using one-way binding instead of bind:value
• Not exporting prop in child component